1. Field of the Invention
The present invention relates to digital processing of a video signal and in particular to an image compressing and enlarging circuitry which is capable of displaying an image on a screen without an apparent disparity between the image and the screen by compressing or enlarging an image to a desired size to match the aspect ratio of the screen of a display even if the aspect ratio of the image obtained by a video signal is different from the aspect ratio of the screen of the display.
2. Description of the Related Art
Test broadcasting of high definition TV started in 1991 in Japan and the home TV sets with a screen having an aspect ratio of 16:9 have been used. Matching the TV sets that have a screen having an aspect ratio of 16:9 with the conventional TV broadcasting is critical. Thus how the aspect ratio of the image of the broadcast is changed into 16:9 is an important problem. If an image having movie size which is commonly found in a number of recent movie software packages, that is, so-called letter box type image (the aspect ratios of the image are various) in which the aspect ratio of the whole image is 4:3 and there are blanking areas on upper and lower sides of a screen is displayed on a display with a screen having an aspect ratio of 16:9, an aspect ratio which is approximate to 16:9 could be obtained by enlarging the image in a vertical direction. Therefore, enlargement of images in a vertical direction has become a critical technology.
JP-A-1-194784 discloses a combined method including a method of adapting an image having an aspect ratio of 4:3 onto a screen having an aspect ratio of 16:9 by changing the write frequency of a line memory and the read frequency thereof to compress the image in a horizontal direction and a method of enlarging the image in a vertical direction by changing the amplitude of an output of the deflecting circuit for cutting the upper and lower areas of the image.
JP-A-3-11891 discloses a technology for enlarging an image in a vertical direction by digital signal processing. The proposed circuit configuration for interpolating scanning lines using digital signal processing to enlarge the image in a vertical direction is shown in FIG. 1.
In FIG. 1, reference numerals 201 and 202 denote input and output terminals of digitized video signal, respectively; 203 a memory having a capacity of at least about 120 lines; 204 one-line memory; 205 and 206 ROM tables which are used for multiplying an input signal by a coefficient; 207 an adder; 208 an input terminal for a control signal for the memory 203; 209 an input terminal for a control signal for switching tables of the ROMs 205 and 206.
In the disclosed invention, the memory 203 is controlled to conduct reading of the same line plural times with a predetermined line period in accordance with a control signal from the input terminal 208. As a result of delay of a video signal by one line in the one-line memory 204, scanning line signals of adjacent upper and lower lines are supplied to the ROMs 205 and 206 and are multiplied by a coefficient in a ROM table which is selected by a control signal from the input terminal 209. A signal having a suitable center of gravity of scanning lines can be obtained from the adder 207.
The thus obtained enlarged image provide a large size picture displayed with high quality since the spacings between scanning lines when progressive scanning signals are used are less changed in comparison with a case in which the image are enlarged by changing the amplitude of the output of the deflecting circuit.
In such a manner, various techniques have been used to display an image having an aspect ratio of 4:3 on a screen having an aspect ratio of 16:9.
The above mentioned two inventions are excellent as systems for displaying an image having an aspect ratio of 4:3 on a screen having an aspect ratio of 16:9. However, letter box type image of movie software which are commonly found in recent software packages have various aspect ratios. It has become harder to change an image having various aspect ratios to an optimum size for displays having an aspect ratio of 16:9.
It is necessary to display a framing signal particularly in projector type displays when most images having an aspect ratio of 4:3 are compressed and displayed. A problem of a sticking of the display screen or a difference in luminescence decay of a phosphor may occur. Accordingly, a method of displaying no framing signal is required.